Field of the Invention
The present invention relates to a removable mask for a platform of a blade or a nozzle sector of a turbine engine, the blade or sector being for covering in an electrolytic deposit.
Description of the Related Art
During fabrication, the blades or the elements making up a high-pressure nozzle of a turbine engine such as an airplane turboprop or turbojet are covered in an electrolytic deposit of an underlayer for bonding an outer layer of ceramic. The ceramic layer needs to be deposited on those outer surfaces of the blades or of the nozzle that are situated in the stream of gas flowing stream through the turbine engine, i.e.: for blades on the pressure and suction sides of the airfoils and on the radially-outer surfaces of the platforms; and for a nozzle, on the pressure and suction sides of vanes, on the radially-inner surfaces of the outer platforms, and on the radially-outer surfaces of the inner platforms. The other outer surfaces of blades or of the nozzle, i.e. the surfaces of their platforms that do not lie in the gas flow stream, are not covered in ceramic, so there is therefore not any need to deposit the above-mentioned bonding underlayer on those surfaces.
The surface of each platform that is to be covered in ceramic is connected via a connecting edge (substantially of parallelogram shape) to a peripheral surface that extends substantially perpendicularly to the above-mentioned surface of the platform and that is not to be covered in ceramic.
In general, the blade or the nozzle is made as a one-piece casting. The surfaces of the blade or of the nozzle that are to be covered in ceramic are not rectified by machining. The above-mentioned peripheral surface of each platform is rectified in order to have accurate dimensions.
The bonding underlayer for the ceramic layer is generally based on platinum, which is a metal that is very expensive. It is therefore important to avoid depositing that substance on those surfaces of the platforms that lie outside the gas flow stream, in order to limit the cost of fabricating the blades or the nozzle.
In the prior art, two technologies are used for masking the above-mentioned surfaces of the platforms of blades or nozzles during an electrolytic deposition operation.
The first technology consists in casing wax directly onto the platforms in order to mask their outer surfaces that are not to be covered in platinum.
The second technology consists in sticking adhesive tapes on the surfaces that are to be masked.
Nevertheless, those technologies are not satisfactory since they are lengthy and expensive to implement and since it is found that platinum manages to infiltrate between the platforms and the adhesive tapes or between the platforms and the wax, in particular because both adhesive tapes and wax deform during electrolytic deposition. Those technologies are also sources of problems of hygiene and safety, in particular because they involve using toxic substances for cleaning together with a blade for cutting the adhesive tapes.
Furthermore, those technologies are not suitable for covering the surfaces for masking in accurate manner, and in particular for masking the above-mentioned peripheral surface of each platform, since the masking means used may either project beyond the outer surface of a platform that is to be covered in an electrolytic deposit, thereby masking a portion of that surface, or on the contrary they may be set back from said surface, thereby leaving free portions of the peripheral surface that then become covered in the electrolytic deposit.
Finally, those technologies do not make it possible to guarantee that the electrolytic deposit is uniform over the outer surface of the platform of a blade or of a nozzle, in particular at its periphery. It is very difficult to position the wax or the adhesive tape on the peripheral surface of the platform in such a manner that the top edge face of the wax or of the tape is flush with and in alignment with the outer surface of the platform that is exposed to hot gas in operation. The top edge face of the wax or of the tape is generally situated a little below or a little above the outer surface of the part, which can lead to infiltrations or to excess electrolytic deposit along the side edge faces of the platform.
The quantity of platinum deposited electrolytically on a nozzle is generally about 4 grams (g) with a tolerance of ±0.15 g. The above-mentioned infiltrations can lead to an increase of electrolytic deposition of platinum of more than 0.15 g, which represents a significant financial loss.